A display capacity of liquid crystal display devices has recently been on the steady increase. There are two types of liquid crystal display devices in constitution, that is, a passive-matrix type wherein display electrodes for liquid crystal pixels are directly connected to signal electrodes installed on a first substrate thereof, and an active-matrix type having switching elements installed between signal electrodes and display electrodes.
Further, the active-matrix type liquid crystal display device has a constitution wherein opposed electrodes are installed opposite to the display electrodes provided on the first substrate with liquid crystals interposed therebetween, and a plurality of the signal electrodes and a plurality of the opposed electrodes are disposed in a matrix fashion such that given signals from an external circuit are applied to respective data electrodes connected to the signal electrodes and the opposed electrodes.
In the case of applying multiplex driving to a liquid crystal display device of a simple matrix constitution (the passive-matrix type), deterioration in contrast or response time occurs as the multiplexing reaches a higher time-division rate, so that it becomes difficult to obtain sufficient contrast when scanning lines in the order of two hundred.
Accordingly, a liquid crystal display panel of the active-matrix type wherein individual pixels are provided with a switching element has been adopted in order to remove such a drawback.
For the switching element in the liquid crystal display panel of the active-matrix type, there are in use a three-terminal type switching element using a thin-film transistor, and a two-terminal type switching element using a nonlinear resistance element. Of these two switching elements, the two-terminal type switching element is regarded as superior in respect of having a simpler construction and an adaptability to fabricate by a relatively low temperature process.
For the two-terminal type switching element, there have been developed a diode-type, a varistor-type, a thin-film-diode (TFD) type, and so forth.
Among these types, the TFD type is characterized especially by simple construction and in addition, by a short-time fabrication process.
Further, since the liquid crystal display device is not a self-light-emitting type display device, display is effected by utilizing an external light source, thereby causing variation in external light due to the optical property of liquid crystals.
Accordingly, the liquid crystal display device is broadly classified into two kinds in terms of relative positions of a viewer, the liquid crystal display device, and the light source. First, there is one wherein the light source (a main light source) and the viewer are on the same side relative to the liquid crystal display device, the so-called reflection-type liquid crystal display device. Second, there is one wherein the viewer, the liquid crystal display device and the light source (the main light source) are disposed in that order, the so-called transmission-type liquid crystal display device.
In the case where an object is to achieve low power consumption, which is the merit of a liquid crystal display device, the reflection-type liquid crystal display device taking advantage of the light source located in the neighborhood thereof without the light source installed therein is more effective.
There is also available a transflective liquid crystal display device functioning as the reflection-type liquid crystal display device taking advantage of the external light source (the main light source) when an application environment thereof is bright while functioning as the transmission-type liquid crystal display device by lighting up an auxiliary light source built therein when the application environment is dark.
Since the transflective liquid crystal display device is employed basically as the reflection-type liquid crystal display device, its power consumption can be lowered in comparison with that for the transmission-type liquid crystal display device. It is for this reason that the reflection-type liquid crystal display device or the transflective liquid crystal display device is a very important display device for application to portable information equipment.
A conventional reflection-type liquid crystal display device having the two-terminal type switching elements as switching elements installed between signal electrodes and display electrodes is described hereinafter with reference to the accompanying drawings.
FIG. 26 is an enlarged plan view showing an electrode constitution at a pixel of the conventional reflection-type liquid crystal display device using the two-terminal type switching element. FIG. 27 is a partial sectional view of the conventional reflection-type liquid crystal display device, taken along line A-A in FIG. 26.
As shown in FIG. 27, with this reflection-type liquid crystal display device, a pair of a first substrate 1 and a second substrate 2, made up of a transparent glass substrate, are disposed opposite to each other with a given gap provided therebetween. On the surface of the second substrate 2, a signal electrode 3, made up of a tantalum (Ta) film, and a lower electrode 4, formed integrally with the signal electrode 3 so as to be projected sideways from the side thereof (refer to FIG. 26) are provided. On the surface of the signal electrode 3 and the lower electrode 4, a nonlinear resistance layer 5 made up of a tantalum oxide (Ta2O5) film is provided.
Further, an upper electrode 6 and a display electrode 9 formed integrally with the upper electrode 6 as shown in FIG. 26, made up of an indium tin oxide (ITO) film which is a transparent and electrically conductive film, are provided so as to overlap the nonlinear resistance layer 5 provided on top of the lower electrode 4. The upper electrode 6, the nonlinear resistance layer 5, and the lower electrode 4 constitute a two-terminal switching element 7.
On the inner face of the first substrate 1, facing the second substrate 2, there is disposed an opposed electrodes 12, formed in stripes and made up of an indium tin oxide (ITO) film which is a transparent and electrically conductive film, enabling it to face the display electrode 9. Further, a data electrode (not shown) for applying signals from an external circuit is connected to the respective opposed electrodes 12.
Further, on the inner faces of the first substrate 1 and the second substrate 2, facing each other, there are provided alignment layers 15A, 15B, respectively, functioning as layers for alignment treatment wherein molecules of liquid crystals 16 sealed in-between the inner faces are aligned in a regular fashion.
The first substrate 1 and the second substrate 2 are disposed so as to face each other across a given gap formed by spacers (not shown), and the liquid crystals 16 are sealed in the gap.
Further, polarizing films 21A, 21B are disposed on the outer faces of the first substrate 1 and the second substrate 2, respectively, and a reflector 25 is disposed on a side of either of the polarizing films 21A, 21B, opposite from the liquid crystal 16 (in an example shown in FIG. 27, on the outer face of the second substrate 2). There is a case where the polarizing films 21A, 21B are required and a case where the polarizing films 21A, 21B are not required, depending on the type of the display mode of the liquid crystal display device, for example, phase-transition type guest-host (p-GH) mode, twisted-nematic (TN) mode, and so forth.
Since no light is emitted by the liquid crystal display device itself, a voltage at driving waveforms is applied from an external circuit to the signal electrodes 3 and the data electrodes not shown (connected to the opposed electrodes). By applying the voltage via the switching elements 7 to the liquid crystal 16 in regions between the display electrodes 9 and the opposed electrodes 12, thereby causing changes in the optical property of the liquid crystal 16 to occur, and in addition, by taking advantage of the reflection characteristics of the reflector 25 and external light 31, a display of desired images is effected.
However, with the conventional liquid crystal display device described above, images displayed had a good contrast ratio but were found lacking in brightness, especially, whiteness, so that its display performance was less than satisfactory. In the case of using color filters, brightness further declined.
Further, in the case where projections and depressions are provided on the surface of the reflector, control of the surface shape thereof and enhancement of reflectance are needed, thus involving complex work in forming the reflector. Furthermore, in the case of employing a white diffusion film having a specific polarizability, it becomes necessary to match the direction of light diffusion having a directivity dependent on the surface shape with the direction of polarization, and consequently, a white diffusion film having dependence on the direction of polarization of the liquid crystal display device needs to be prepared, so that general versatility is impaired.
Also, with the reflection-type liquid crystal display device wherein losses in light quantity occur due to installation of the polarizing films, it becomes necessary to enhance brightness by utilizing the light diffusion film in combined use of the polarizing films and the reflector such that losses in light quantity can be prevented as much as possible.
Furthermore, in the case where the liquid crystal display device is provided with an auxiliary light source, it is necessary to enhance brightness by utilizing the white diffusion film in combined use of the auxiliary light source and the reflector.
The invention has been developed to solve the problems described above, and an object of the invention is to provide a reflection-type liquid crystal display device capable of effecting bright and whitish display.